Multiplicity and transverse momentum dependence of two- and four-particle correlations in pPb and PbPb collisions

We present results for the elliptic and triangular flow coefficients in Au+Au collisions at root-s=200 AGeV using event-by-event (3+1)D viscous hydrodynamic simulations. We study the effect of initial state fluctuations and finite viscosities on the flow coefficients v_2 and v_3 as functions of transverse momentum and pseudo-rapidity. Fluctuations are essential to reproduce the measured centrality dependence of elliptic flow. We argue that simultaneous measurements of v_2 and v_3 can determine eta/s more precisely.

We introduce a cumulant expansion to parameterize possible initial conditions in relativistic heavy ion collisions. We show that the cumulant expansion converges and that it can systematically reproduce the results of Glauber type initial conditions. At third order in the gradient expansion, the cumulants characterize the triangularity \( \) and the dipole asymmetry \( \) of the initial entropy distribution. We show that for mid-peripheral collisions the orientation angle of the dipole asymmetry \(\psi_{1,3}\) has a \(20%\) preference out of plane. This leads to a small net \(v_1\) out of plane. In peripheral and mid-central collisions the orientation angles \(\psi_{1,3}\) and \(\psi_{3,3}\) are strongly correlated. We study the ideal hydrodynamic response to these cumulants and determine the associated \(v_1/\epsilon_1\) and \(v_3/\epsilon_3\) for a massless ideal gas equation of state. \(v_1\) and \(v_3\) develop towards the edge of the nucleus, and consequently the final spectra are more sensitive to the viscous dynamics of freezeout. The hydrodynamic calculations for \(v_3\) are compared to Alver and Roland fit of two particle correlation functions. Finally, we propose to measure the \(v_1\) associated with the dipole asymmetry and the correlations between \(\psi_{1,3}\) and \(\psi_{3,3}\) by measuring a two particle correlation with respect to the participant plane, \( \). The hydrodynamic prediction for this correlation function is several times larger than a correlation currently measured by the STAR collaboration, \( \).